[MPArch] Remove deprecated func usage in a SitePerProcessWebViewTest

This CL re-expresses the SitePerProcessWebViewTest.Shim_BlankWebview test
in terms of RenderFrameHost to avoid using deprecated function
DeprecatedWaitForSingleGuestCreated() in favor of its RenderFrameHost
version.

Bug: 1261928
Change-Id: I13786fb16f3af8a80fec83745fa109e85b8088c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3861333
Commit-Queue: Antia Puentes <apuentes@igalia.com>
Reviewed-by: Kevin McNee <mcnee@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1040975}

chrome/browser/apps/guest_view/web_view_browsertest.cc

@@ -5982,11 +5982,11 @@ IN_PROC_BROWSER_TEST_F(SitePerProcessWebViewTest, NavigateToAboutBlank) {
 IN_PROC_BROWSER_TEST_F(SitePerProcessWebViewTest, Shim_BlankWebview) {
   TestHelper("testBlankWebview", "web_view/shim", NO_TEST_SERVER);
 
-  content::WebContents* guest =
-      GetGuestViewManager()->DeprecatedWaitForSingleGuestCreated();
-  ASSERT_TRUE(guest);
+  content::RenderFrameHost* guest_rfh =
+      GetGuestViewManager()->WaitForSingleGuestRenderFrameHostCreated();
+  ASSERT_TRUE(guest_rfh);
   scoped_refptr<content::SiteInstance> site_instance =
-      guest->GetPrimaryMainFrame()->GetSiteInstance();
+      guest_rfh->GetSiteInstance();
   EXPECT_TRUE(site_instance->IsGuest());
   EXPECT_TRUE(site_instance->GetProcess()->IsForGuestsOnly());
 }